1. Field of the Invention
The present invention relates to an elevator control apparatus, and more particularly to an elevator control apparatus which compensates for an insufficient gain of the entire control system caused by a drop in power source voltage.
2. Description of the Related Art
Elevator control apparatuses perform highly accurate speed control in such a manner that, generally, the apparatus detects the speed of a car, compares the detected speed value with a speed command value, and returns to a speed control system in the form of feedback, a signal showing the difference between the two values thereby obtained.
In elevators utilizing a three-phase induction motor, it is required that the control system be stabilized to perform better speed control in consideration of various disturbance factors, such as variations in power source voltage.
FIG. 6 is a schematic diagram illustrating a conventional speed control circuit for an elevator disclosed in Japanese Patent Laid-Open No. 60-6574.
In FIG. 6, a speed control circuit 60 includes a subtracter 61 for calculating the difference between a speed command signal VP and a speed detecting signal VT, and a compensator 62 for compensating for, based on a deviation output "e" of the subtracter 61, the gain and phase properties of the control system.
The compensator 62 is composed of an analog circuit, and one having a transfer function G(S) usually expressed in the following equation is utilized. ##EQU1##
where K is a gain, T1 and T2 are time constants, and S is a Laplace operator.
In such a conventional speed control circuit as constructed above, to improve riding comfort and precision of stopping at a floor, the compensator 62 compensates for gain as well as phase, and the ignition angle of an unillustrated thyristor is controlled by adding the compensated voltage signal V0 to the thyristor, as a control signal.
There are problems in that since the generating torque of an induction motor in the conventional elevator control apparatus as described above depends generally upon the input voltage and, in detail, is proportional to the square of the input voltage, the gain of the entire control system diminishes if the voltage drops owing to variations in the power source voltage or the like. With this diminution, the response of the control system to a speed command markedly decreases, thereby causing inaccuracy of stopping of a car at a floor or riding discomfort.
In order to overcome the above problems, it is possible to keep the gain of the entire control system constantly high, allowing for variations in the power source voltage or the like. However, a problem exists in that this makes the control system unstable and the car easily vibrates, thereby causing riding discomfort. Moreover, setting of a gain corresponding to variations in the power source voltage or the like demands a detecting circuit for detecting the variations in the power source voltage or the like, resulting in raised cost as well as complicated hardware structures.